'\" et
.TH PTHREAD_ATTR_GETGUARDSIZE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
.ad l
pthread_attr_getguardsize,
pthread_attr_setguardsize
\(em get and set the thread guardsize attribute
.ad b
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
int pthread_attr_getguardsize(const pthread_attr_t *restrict \fIattr\fP,
    size_t *restrict \fIguardsize\fP);
int pthread_attr_setguardsize(pthread_attr_t *\fIattr\fP,
    size_t \fIguardsize\fP);
.fi
.SH DESCRIPTION
The
\fIpthread_attr_getguardsize\fR()
function shall get the
.IR guardsize
attribute in the
.IR attr
object. This attribute shall be returned in the
.IR guardsize
parameter.
.P
The
\fIpthread_attr_setguardsize\fR()
function shall set the
.IR guardsize
attribute in the
.IR attr
object. The new value of this attribute shall be obtained from the
.IR guardsize
parameter. If
.IR guardsize
is zero, a guard area shall not be provided for threads created with
.IR attr .
If
.IR guardsize
is greater than zero, a guard area of at least size
.IR guardsize
bytes shall be provided for each thread created with
.IR attr .
.P
The
.IR guardsize
attribute controls the size of the guard area for the created thread's
stack. The
.IR guardsize
attribute provides protection against overflow of the stack pointer. If
a thread's stack is created with guard protection, the implementation
allocates extra memory at the overflow end of the stack as a buffer
against stack overflow of the stack pointer. If an application
overflows into this buffer an error shall result (possibly in a SIGSEGV
signal being delivered to the thread).
.P
A conforming implementation may round up the value contained in
.IR guardsize
to a multiple of the configurable system variable
{PAGESIZE}
(see
.IR <sys/mman.h> ).
If an implementation rounds up the value of
.IR guardsize
to a multiple of
{PAGESIZE},
a call to
\fIpthread_attr_getguardsize\fR()
specifying
.IR attr
shall store in the
.IR guardsize
parameter the guard size specified by the previous
\fIpthread_attr_setguardsize\fR()
function call.
.P
The default value of the
.IR guardsize
attribute is implementation-defined.
.P
If the
.IR stackaddr
attribute has been set (that is, the caller is allocating and managing
its own thread stacks), the
.IR guardsize
attribute shall be ignored and no protection shall be provided by the
implementation. It is the responsibility of the application to manage
stack overflow along with stack allocation and management in this
case.
.P
The behavior is undefined if the value specified by the
.IR attr
argument to
\fIpthread_attr_getguardsize\fR()
or
\fIpthread_attr_setguardsize\fR()
does not refer to an initialized thread attributes object.
.SH "RETURN VALUE"
If successful, the
\fIpthread_attr_getguardsize\fR()
and
\fIpthread_attr_setguardsize\fR()
functions shall return zero; otherwise, an error number shall be
returned to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EINVAL
The parameter
.IR guardsize
is invalid.
.P
These functions shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Retrieving the guardsize Attribute"
.P
This example shows how to obtain the
.IR guardsize
attribute of a thread attribute object.
.sp
.RS 4
.nf

#include <pthread.h>
.P
pthread_attr_t thread_attr;
size_t  guardsize;
int     rc;
.P
/* code initializing thread_attr */
\&...
.P
rc = pthread_attr_getguardsize (&thread_attr, &guardsize);
if (rc != 0)  {
    /* handle error */
    ...
}
else {
    if (guardsize > 0) {
    /* a guard area of at least guardsize bytes is provided */
    ...
    }
    else {
    /* no guard area provided */
    ...
    }
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
The
.IR guardsize
attribute is provided to the application for two reasons:
.IP " 1." 4
Overflow protection can potentially result in wasted system resources.
An application that creates a large number of threads, and which knows
its threads never overflow their stack, can save system resources by
turning off guard areas.
.IP " 2." 4
When threads allocate large data structures on the stack, large guard
areas may be needed to detect stack overflow.
.P
The default size of the guard area is left implementation-defined
since on systems supporting very large page sizes, the overhead
might be substantial if at least one guard page is required by default.
.P
If an implementation detects that the value specified by the
.IR attr
argument to
\fIpthread_attr_getguardsize\fR()
or
\fIpthread_attr_setguardsize\fR()
does not refer to an initialized thread attributes object, it is
recommended that the function should fail and report an
.BR [EINVAL] 
error.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<pthread.h>\fP",
.IR "\fB<sys_mman.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
